Properly restore the LC_NUMERIC locale after finalizing.

There were two issues here:

 1. setlocale(LC_NUMERIC, "C") always returns C, not the previous
    locale.
 2. The return value of setlocale() may point into static storage,
    which may be corrupted when we call into libGL, if e.g.
    the shader compiler calls setlocale() on its own.

Patch by Jean-Baptiste Mardelle.